Materials, method and apparatus for detection and monitoring of chemical species

ABSTRACT

A method for investigating a target environment to determine whether or in what amount a chemical species may be present therein, which comprises: (a) exposing to said environment an article of manufacture comprising a multiplicity of particles in close-packed orientation, said particles having a core of conductive metal or conductive metal alloy and deposited thereon a ligand which is capable of interacting with said species such that a property of said multiplicity of particles is altered; (b) subjecting said multiplicity of particles to conditions sufficient for said property to be exhibited; and (c) monitoring said property to determine whether there is, or the amount of, any change as an indication of whether, or in what amount, said species is present; a multiplicity of particles suitable for use in such method; and equipment suitable for implementing the method.

This application claims the benefit of U.S. Provisional No. 60/069,763filed Nov. 25, 1997.

FIELD OF THE INVENTION

The invention relates to the detection or quantitation of a chemicalspecies in a target environment, and the discovery that the propertiesof certain particles which are interactive with the chemical species canadvantageously be monitored as a indication or in which amount thespecies is present.

BACKGROUND OF THE INVENTION

There are a number of known approaches to determining the presence oramount of a chemical species in a target environment by exposing asubstance capable of interacting with the species to such environmentand monitoring a change in a property of the substance due to suchinteraction as an indication of whether or in what amount the species ispresent.

One such approach has been the exposure to the environment of aspecies-interactive substance applied to a piezoelectric substrate. Thesubstance is affected such that, if any of the species present, apreselected property of the substance is changed. A surface acousticwave is induced in the piezoelectric material. Any change of property inthe substance results in an attenuation of the surface acoustic wave,which can be monitored as an indication of whether or in what amount thespecies is present. For instance, see U.S. Pat. Nos. 4,312,228 and4,759,210.

Another approach has been the provision of a capacitive device fordetecting the presence or measuring the concentration of an analyte in afluid medium. A plurality of interdigitated fingers formed from metallicconductors are placed upon an insulating substrate. The substrate may bemade from an insulating material such as glass and the fingers may bemade of copper and gold; the fingers are covered with an insulatingpassivation layer. The approach involves biospecific binding between abiochemical binding system and the analyte to change the dielectricproperties of the sensor. See U.S. Pat. No. 4,822,566.

Yet another approach has been the provision of a chemical sensorcomprising a thin film of dithiolene transition metal complexes appliedto a chemiresistor device. The film is deposited upon an interdigitatedelectrode on a substrate. The film changes conductivity when exposed toa chemical gas or vapor of analytical interest. The interdigitatedelectrodes may be gold and the substrate is an insulating material suchas quartz. A power supply and current measuring device are included. SeeU.S. Pat. No. 4,992,244.

Still another approach has been provision of a biosensor in the natureof a sample testing device that includes an electrode structure whichmakes measurements of one or more electrically measurablecharacteristics of the sample. The area between two electrodes on onewall of the test cell can be coated with a binding agent which can bindconducting particles such as gold sol particles. See U.S. Pat. No.5,141,868.

A different type of biosensor which has also been suggested has a thincrystalline drive surfactant polymeric electrically conducting layer towhich may be bound members of specific binding pairs. Binding of ananalyte or reagent to the binding pair member layer may changeelectrical properties of the layer for measurement of the analyte. SeeU.S. Pat. No. 5,156,810.

However, it would still be desirable for the art to have an alternativedetection technology which lends itself to ready and versatileimplementation as well as consumes power at a very low level, withoutsacrificing reliability, miniaturization affinity, and low cost.

OBJECTS OF THE INVENTION

It is an object of the invention to provide sensitive and reliabletechnology for the detection and monitoring of chemical species.

It is another object of the invention to provide materials, methods andequipment suitable for the sensitive and reliable detection orquantitation of a preselected chemical species in a target environment.

It is yet another object of the invention to provide materials, methodsand equipment as aforesaid which are well-suited for applicationsrequiring compact size, low cost and low power consumption.

It is a further object of the invention to provide methods offabricating the aforementioned equipment.

SUMMARY OF THE INVENTION

In one aspect, the invention is in an article of manufacture suitablefor use in determining whether or in what amount a chemical species ispresent in a target environment, which article comprises a multiplicityof particles in close-packed orientation, said particles having a coreof conductive metal or conductive metal alloy, in each said particlesuch core being of 0.8 to 40.0 nm in maximum dimension, and on said corea ligand shell, of thickness from 0.4 to 4.0 nm, which is capable ofinteracting with said species such that a property of said multiplicityof particles is altered.

In a further aspect, the invention is in a method of investigating atarget environment to determine whether or in what amount a chemicalspecies may be present, which comprises: (a) exposing to saidenvironment an article of manufacture comprising a multiplicity ofparticles in close-packed orientation, said particles having a core ofconductive metal or conductive metal alloy and deposited thereon aligand which is capable of interacting with said species such that aproperty of said multiplicity of particles is altered; (b) subjectingsaid multiplicity of particles to conditions sufficient for saidproperty to be exhibited; and (c) monitoring said property to determinewhether there is, or the amount of, any change as an indication ofwhether, or in what amount, said species is present.

In another aspect, the invention is in an assembly suitable forinvestigation of a target environment to determine whether or in whatamount a chemical species may be present, which comprises: (a) asubstrate suitably configured for presenting a multiplicity of particlessupported thereon to contact with said environment; (b) supported bysaid substrate, a multiplicity of particles in close-packed orientation,said particles having a core of conductive metal or conductive metalalloy and deposited thereon a ligand which is capable of interactingwith said species such that a property of said multiplicity of particlesis altered; and (c) a sensor for monitoring said property of saidmultiplicity of particles.

In yet another aspect the invention is in a method of fabricating anassembly suitable for investigation of a target environment to determinewhether or in what amount a chemical species may be present, whichcomprises depositing on a substrate (i) a pair of interdigitatedelectrodes each having a comb-like configuration and (ii) in such mannerthat the electrodes are electrically connected, a thin film of amultiplicity of particles having a core of conductive metal orconductive metal alloy, in each said particle the core being from 0.8 to40.0 nm in maximum dimension, and deposited on said core a ligand shell,of thickness from 0.4 to 4.0 nm, which is capable of interacting withsaid species such that a property of said multiplicity of particles isaltered.

In still a further aspect the invention in a system suitable forinvestigating a target environment to determine whether or in whatamount a chemical species may be present, which comprises: (a) amultiplicity of particles in close-packed orientation, said particleshaving a core of conductive metal or conductive metal alloy anddeposited thereon a ligand which is capable of interacting with saidspecies such that a property of said multiplicity of particles isaltered; (b) means for exposing said multiplicity of particles to saidenvironment; (c) means for subjecting said multiplicity of particles toconditions sufficient for said property to be exhibited; and (d) meansfor monitoring said property to determine whether there is, or theamount of, any change in such property as an indication of whether or inwhat amount said species is present.

By the term “close-packed orientation” herein we mean a solid-statearrangement of nanoscale metal particles as aforesaid, wherein ligandshells are in contact with their nearest neighbors and in whichmultimolecular sized (for example, 0.1 to 1.0 nm) voids among theparticles are interconnected. A particle is typically “stabilized” inthat the metal core of the particle is effectively encapsulated by theligand shell.

Practice of the invention results in substantial advantages. Themultiplicity of particles is effective in extremely small amounts, andthus the method and equipment embodiments which utilize suchmultiplicity of particles can be implemented on a commensurately smallscale. Furthermore, the multiplicity of particles is extremely sensitivein detection applications. Additionally, by varying the ligandcomponent, the metal core size or type and the ligand shell size ortype, the multiplicity of particles can be made highly sensitive tointeraction with, and thus detection of, a wide range of chemicalspecies. Consequently, the method and equipment embodiments whichinvolve such multiplicity of particles are also highly sensitive andhighly adaptable. Moreover, fabrication and use of a sensor assembly orsystem in accordance with the invention is relatively simple andrelatively inexpensive; for instance, the sensor assembly can readily beinterfaced with inexpensive electronic read-out devices, and itsoperation requires very little electrical power. Also, due to simplicityof design, the sensor assembly and system are rugged and dependable.Other design-based benefits are the achievability of rapid, linear andreversible response, and versatility such that the environmentinvestigated and the chemical species to be detected via the inventioncan be solid (e.g., porous solid), liquid or gaseous. And, when usingembodiments which incorporate both a sensor component and a referencecomponent, the invention further is temperature-compensated and tolerantof production variations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of particles arranged in close-packedorientation according to the invention.

FIG. 2 is a schematic depiction of a basic sensor system in accordancewith the invention.

FIG. 3 is a schematic depiction of another sensor system according tothe invention, which system includes both a sensor component and areference component.

FIG. 4 is a schematic depiction of a sensor system in accordance withthe invention.

FIG. 5 is a plot of signal v. time response showing the variation inresistivity, upon exposure to toluene vapor, for the system depicted inFIG. 4.

FIG. 6 is a plot of toluene content v. resistivity showing change in thelatter which results from change in toluene content for the systemdepicted in FIG. 4.

FIG. 7 is a plot of signal v. time response as a function of thepresence of toluene for the system depicted in FIG. 4.

FIG. 8 is a plot of chemical species content v. resistivity showing thechange in resistivity which results from a change in content of severaldifferent substances for the system depicted in FIG. 4.

FIGS. 9A and 9B are, respectively, a plot of ligand shell thickness ofvarious particles v. the response of a multiplicity of such particles tochanges in toluene content, and a plot of particle core radius forvarious particles v. the response of a multiplicity of such particles tochanges in toluene content.

DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

A central feature of the invention is a multiplicity of particles inclose-packed orientation, as aforesaid. Each of the particles is anextremely small cluster of conductive metal atoms that forms a metallic“core” surrounded by a thin “ligand shell” of relatively non-conductivematerial chemically (e.g., covalently) bound to the core.

The cluster of metal atoms can be composed of a single conductive metal,or of atoms of two or more conductive metals. Suitable conductive metalsare metals capable of being processed on a nanoscale and of bonding to athin insulating ligand shell to form a stabilized metal particle, amultiplicity of which particles is stable in respect of ambientenvironments and exhibits a stable and measurable electricalconductivity. Examples are noble metals or other conductive metals suchas copper, nickel and tin. The elemental metal core is illustratively anoble metal, preferably silver, gold, platinum or palladium. The metalalloy core is illustratively a combination of two or more noble metals,such as two or more of silver, gold, platinum and palladium. The corebodies are advantageously spherical or spheroidal, though they can alsobe of other regular shapes, or irregular in shape; as will be apparentthe shape of the particle typically simulates the shape of the core.Also, typically, the metal cluster core will range from 0.8 to 40 nm(preferably 2 to 20 nm) in maximum dimension; the core is advantageouslyspherical, in which case its typical size is radius of from 0.4 to 20nm, preferably 1 to 10 nm.

The encapsulating ligand shell is advantageously an organic, inorganicor combined organic/inorganic substance which is preselected for itsability to interact with the chemical species of interest such that theligand shell is changed in a manner perceptibly affecting a property ofthe multiplicity of particles, with the result that the species can bedetected if present. The ligand molecule typically has a head-tail typestructure; the head is a functional group possessing a bondinginteraction with metal atoms in the core surface, and the tail has astructure and composition designed to provide additional stabilizationof metal clusters (i.e., core bodies) against irreversibleagglomeration, induce solubility in solvents and promote interactionswith chemical species of interest. The ligand shell can be amonomolecular or multimolecular layer. The ligand shell substance isadvantageously a functionalized organic compound, such as a thiol, or anamine. These thiols can be primary aliphatic thiols (preferably straightchain or branched), secondary aliphatic thiols, tertiary aliphaticthiols, aliphatic thiols substituted heterofunctionally (for instance,by OH, COOH NH₂, Cl, and the like, preferably HS(CH₂)₆OH or thehexafluoroacetone adduct) aromatic thiols, aromatic thiols substitutedheterofunctionally (for instance, by OH, COOH NH₂, Cl, and the like,preferably HS(CH₂)₆OH or the hexafluoroacetone adduct) and araliphaticthiols substituted heterofunctionally (for instance, by OH, COOH NH₂,Cl, and the like, preferably HS(CH₂)₆OH or the hexafluoroacetoneadduct). Preferred amines are primary aliphatic amines. The aliphaticportions of such thiols and amines can be of from 3 to 20 carbon atoms,especially 4 to 16 carbon atoms.

The shell is advantageously neither so thin that the multiplicity ofparticles is effectively metallic in its conductivity properties, nor sothick that the multiplicity of particles is completely electricallyinsulating. Preferably, such thickness ranges from 0.4 to 4 nm,especially 0.4 to 2.5 nm. The organic ligand shell stabilizes the metalcluster against irreversible coagulation and also imparts a highsolubility of the cluster complex in organic solvents. This allows for aprocessibility of these materials as thin films as discussedhereinafter.

Once in possession of the teachings herein, one of ordinary in the artwill be able to prepare the subject particles by dissolving a salt ofthe conductive metal—or in the case of an alloy, salts of the conductivemetals—of which the core is to be composed, and an organic substancecorresponding to the desired ligand, in a common solvent andsubsequently adding a reducing agent under conditions of rapid mixing(see M. Brust et al., J. Chem. Soc., Chem. Comm. 1994, 801; D. V. Leffet al, Langmuir 1996, 12, 4723). The metal ions of the salt(s) arereduced to neutral atoms and subsequently nucleate to form multiatomcore bodies. These core bodies grow by absorption of additional metalatoms. Competitively, the organic ligand molecule is absorbed on thegrowing metal core body surface, encapsulates the metal core body andterminates its growth. The relative concentrations of the metal saltsand organic ligand molecules determine the relative rates of metal corebody growth and organic ligand encapsulation, and thus the size of themetal core in the stabilized particle. The thickness of the ligand shellis determined by the size of the ligand molecule. It is important thatthere be a strong chemical interaction between the ligand molecule andneutral metal otherwise the metal core bodies will coagulate and notredisperse. The choice of a suitable ligand molecule is within the skillof the art once the practitioner is in possession of the teaching setforth herein. By way of example, sulfur compounds are particularlyeffective for coordination to gold, silver, platinum and copper metals.Amines have a weaker but sufficient interaction with gold. In principle,any combination of reducible metal ion and organic ligand, with asufficient neutral metal to ligand chemical interaction, can form coatedmetal clusters—i.e., particles—useful in this invention. In otherembodiments of the invention alternative synthetic methods can beutilized. For instance, the metal ion reduction can be conductedinitially and the deposition of the ligand shell thereafter. This caninvolve generation of the metal particles in vacuo or in liquidsuspension with subsequent formation of the ligand shell by addition ofthe ligand shell molecules.

A chemical sensor assembly is made when a large ensemble (i.e., amultiplicity) of these particles in close-packed orientation, asaforesaid, is deposited onto a surface equipped with a sensor component,for example, a pair of electrical contacts. The resulting “device” canbe described as a series of metal-insulator-metal (MIM) junctions, wherethe metal is the core and the insulator is the ligand shell. Sometimesherein the invention is referred to as a “metal-insulator-metalensemble” (“MIME”) device. A multiplicity of particles in close-packedorientation is shown in FIG. 1. Each particle has a conductive metalcore 12 and a ligand shell 14, with void space 16 interspersed among theparticles. Variations in the core size and ligand shell thicknessproduce significant changes in the sensitivity to chemical species.

Both the target environment which is being investigated, and thechemical species of interest if present, can be in the vapor, liquid orsolid phase. In this connection, it should be noted that the environmentcan be a pure substance (e.g., if the environment is entirelyconstituted of the chemical species or is devoid of the chemicalspecies) or a combination of substances (e.g., a multi-component gas,liquid or solid system, or a heterogeneous system containing substancesin two or more different phases). When the multiplicity of particles iscontacted with such environment the ligand material interacts with—forinstance, sorbs—the species if it is present. This causes a change inthe ligand material and a change in one or more properties of themultiplicity of particles. Variations in the chemical composition of theligand shell produce significant changes in chemical selectivity. Anyone of a variety of properties of the multiplicity of particles can bemonitored, the only requirement being that the property change as afunction of the interaction of the ligand material with the chemicalspecies (if present). In several good embodiments of the invention anelectrical property, such as conductivity, is monitored. In other goodembodiments, an optical property is monitored (due to the regularspacing of reflective metal centers, optical property changes shouldresult from the swelling of the particles as a consequence ofinteraction with the chemical species). The identification of propertiesto be monitored is within the skill of the art, once those in the artare in possession of the teachings herein.

Formation of the multiplicity of particles as a thin film is a sensitiveoperation. In our experience, casting from solution with slowevaporation does not produce a thin film with reproducibility oracceptable uniformity. We have invented various methods that addressthis problem. For example, a first technique is spraying a solution as afine mist from an air brush onto a substantial surface which has beenheated, preferably to a temperature above the boiling point of thesolvent. The solvent is flashed away leaving a film of very fineuniformly dispersed features. Without heating the features of the filmare very coarse and nonuniform. Another technique is based on chemicalself-assembly. The sensor surface and substrate are cleaned by a plasmaor chemical treatment and coupling agents are applied. Coupling agentsare difunctional molecules with an inert spacing structure separatingthe functional groups (e.g. an a-ω silyl alkanethiol, such as(CH₃O)₃Si(CH₂)₃SH, or a dithiol, HS(CH₂)₆SH)). One functional groupbonds to the sensor/substrate (e.g., the —Si(OCH₃)₃ or the —SHfunctional group) surface, and the other (e.g., a second —SH functionalgroup) is oriented away from the surface for subsequent bonding with themultiplicity of particles. The ligand shell of the metal particle is adynamic system where an individual molecule may be displaced by asimilarly functionalized molecule. Thus, the immobilized thiol group ofthe absorbed coupling agent may bond to a particle and immobilize it onthe aforementioned surface. In this fashion a monolayer of particles ischemisorbed on the surface. Subsequently, the immobilized particlemonolayer is exposed to a solution of a dithiol coupling agent. Thedithiol exchanges with some of the monofunctional thiol ligand moleculesin the immobilized particle ligand shell and positions the second thiolgroup on the outer surface of the immobilized particle's ligand shell. Asecond exposure to a solution for forming the stabilized multiplicity ofparticles results in chemisorption of a second particle layer on thefirst. In this manner many layers of particles are built up into amultilayer film. This offers a very highly controlled, uniform andreproducible deposition where variations in the character of metal coreand ligand shell molecule may be made at any desired depth in thecluster multilayer. As an additional benefit, this film is not removableby solvents or by mild abrasion unlike that of the first technique.

In one basic preferred embodiment the invention provides an assemblyutilizing the aforementioned thin film which assembly comprises a pairof electrodes connected by a thin film of the MIME material. Such anarrangement is shown in FIG. 2. An MIME thin film 20 is formed on asubstrate 22 so as to connect two electrodes 24, 26 also formed thereon.A circuit 28 including a battery 30 and current meter 32 is used tomeasure the electrical conductivity of the MIME device. Conductivity canbe measured under AC or DC conditions.

An even more preferred embodiment is a pair of lithographicallypatterned (e.g., via photolithography, silk-screening, etc.) devices.The pattern results in planar interdigital “comb” electrodes having alarge ratio of electrode perimeter to electrode spacing. The largelength-to-cross-sectional-area ratio serves to decrease the electricalresistance of the device, thereby making it very easy to measure anyconductivity variation with low noise and high precision. In a furtherpreferred embodiment, there is a matched pair of these interdigitaldevices, fabricated simultaneously on the same substrate. As shown inFIG. 3, sensor device 40 and reference device 42 are formed on substrate44. Each such device comprises a pair of interdigitally orientedelectrodes. A multiplicity of particles in accordance with the inventionis deposited between and in contact with the electrodes of each pair.The devices are connected in a circuit 48 containing a voltage source 50and resistors 52, 54, and the output signals from the devices fed to acomparator 56. Sensor device 40 is exposed to an environment containingchemicals to be measured (e.g., gas, liquid or solid), and referencedevice 42 is covered with a passivating layer (e.g., plastic, glass,paraffin wax, etc.), or in some other way is isolated from theenvironment possibly containing the chemical species of interest. Thereference device provides a means to compensate for the normal change inresistance with temperature exhibited by the MIME in thin filmapplications. By fabricating the sensor and reference devicessimultaneously, one is assured of accurate matching and highlyreproducible system performance even if the lot-to-lot productionprocess varies substantially.

In a typical MIME device film thickness is 5 to 10,000 nm. Thecomposition of the MIME thin film and the geometry of the electrode onwhich it is deposited, are selected to provide an attractive baselineresistance (e.g., 10,000 Ω) to permit high precision measurement ofconductivity changes with simple electronic circuitry. Exposure of thefilm to low concentrations (e.g., 100 ppm by volume) of chemical vaporscan change the conductivity by several percent. This is a very largechange if temperature effects can be nulled out by using a referencedevice such as the one disclosed in this invention. Indeed, it is quiteeasy to measure resistance change in 1 part of 10⁵ (i.e., ±0.001%) withthis sensor/reference combination. Thus, monitoring trace level (e.g.sub-ppm) concentrations of many vapors is readily achievable.

The linearity and sensitivity of the MIME sensor are illustrated inFIGS. 5 through 8.

FIG. 4 illustrates a testing apparatus 60 implementing a MIME sensor 64according to the present invention. Apparatus 60 includes a square wavegenerator 62, a MIME sensor 64, a current-to-voltage converter 66, avoltage rectifier 68, a filter 70, and a voltage-to-frequency converter72.

Square wave generator 62 is a conventional wave generator for producinga square waveform signal preferably at a frequency of 100 Hz and havingan amplitude of less than 0.5v. Current-to-voltage converter 66 is aconventional device for converting a supplied current into a voltagepreferably having a range between −5v and +5v. Preferably, the output ofconverter 66 changes by 1 mv in response to each nanoampere of change ininput current. Voltage rectifier 68 is a conventional rectifier devicepreferably adapted for half-wave rectification. Filter 70 is aconventional filter device for filtering transients from a signal and ispreferably comprised of a low-pass filter. Voltage-to-frequencyconverter 72 is a conventional voltage-to-frequency conversion deviceand preferably produces a 350 Hz change in output frequency in responseto each millivolt of change in input voltage. The output signal ofconverter 72 is detected at output node 74.

MIME sensor 64 comprises particles of gold core having a nominal radiusof 1 nanometer combined with an alkanethiol ligand shell having 8 carbonatoms in the alkane chain. A film of this core-ligand materialapproximately 0.4 micrometers in thickness was sprayed onto a smallinterdigital electrode component consisting of 50 “finger pairs” of gold15 μm wide and spaced 15 μm from the next element to form sensor 64. Thefinger aperture was 4800 μm. The gold microelectrode component waslithographically fabricated on an insulating substrate of quartz.

In operation, MIME sensor 64 was excited with an AC square wave havingan amplitude of less than 0.5 V and a frequency of 100 hz produced bygenerator 64. Current flowing through sensor 64 was converted into avoltage by current-to-voltage converter 66. The resulting AC voltage wasrectified by rectifier 68, filtered by filter 70, and then presented tovoltage-to-frequency converter 72. Converter 72 produced an outputsignal which could be detected at node 74. Thus, AC conductance changesin MIME sensor 64 were converted into frequency changes that were easilyrecorded using a laboratory data acquisition system. This approach isnot necessary in order to practice this invention but was convenient forour particular laboratory set-up.

The relative resistance change of the device upon exposure to toluenevapor is shown in FIG. 5. The responses are rapid and reversible. FIG. 6shows the highly linear behavior of the device over a wide range ofconcentrations. FIG. 7 shows the ability of the MIME sensor to detecttrace quantities (e.g., sub parts-per-million by volume concentration)of toluene vapor. FIG. 8 compares the sensitivity of the device to avariety of other vapors. Of great significance is the extraordinarilylow response to water vapor which is essential for practical applicationin which trace levels of organic vapors must be detected and monitoredin humid ambient air.

The sensitivity of the MIME device is related to the nanometer size ofthe encapsulated metal cores, the overall size of which should not begreater than 48 nm in maximum dimension. The size of the metal core andthickness of the ligand shell jointly determine the achievablesensitivity. As an example, FIGS. 9A and 9B display two plots relatingtoluene vapor sensitivity to these two parameters, respectively, for anormal alkanethiol stabilized gold core system (abbreviated Au:SCn(X:Y),wherein n refers to the number of carbon atoms in the normal alkanethiolligand molecule and (X:Y) refers to the gold:alkanethiol stoichiometryused in the synthesis). The upper plot presents the toluene vaporresponse (at a vapor concentration corresponding to P/Po=0.1) of devicesprepared with three homologous series of clusters having constant metalcore radii (determined by the (X:Y) where (1:1), (3:1) and (5:1)correspond to 1.11 nm, 1.57 nm and 2.97 nm, respectively) and varyingligand shell thickness. The maxima of this plot show that each metalcore size has an optimum ligand shell thickness for highest sensitivity.This maximum sensitivity gradually diminishes as one progresses tohomologous series of larger radii. The lower plot of FIG. 9 presents thetoluene vapor response of devices prepared with homologous series ofclusters having constant ligand shell thicknesses (determined by Cnwhere C6, C8, C12 and C16 correspond to 0.71 nm, 0.86 nm, 1.16 nm and1.58 nm, respectively) and varying core radii. This plot also indicatesthat maximum sensitivity correlates with an optimum core size and ligandshell combination and that the overall sensitivity gradually diminishesas the core size progresses to 20 nm. Typically, the range of effectivedimensions for the stabilized metal clusters is 0.4 to 20 nm for thecore radius and 0.4 to 4 nm for the ligand shell thickness.

Sensitivity can also be chemically modulated by introduction of aheterofunctionality to the ligand shell or by adding a binder withheterofunctionality just prior to the deposition of the film. Theheterofunctionality is a heteroatom functional group such as amine,hydroxyl, halogen, phosphoryl, carboxy, ether, etc. In the former case,the ligand molecule is then bifunctional, one functional group to bindwith the metal core surface and the other to provide an attractiveinteraction for sorption of target species. In the latter case, a bindersuch as a polymer is codissolved with the particle component precursorsand simultaneously deposited on the sensor and substrate. Theheterofunctionality in the binder then provides the attractiveinteraction for sorption of target vapors as in the former case above.

The following test procedures and parameters are useful in practice ofthe invention:

Weight Fraction Metal

A thermogravimetric analysis (TGA) measurement of the metal weightfraction in the core/shell composite is conducted using standard thermalanalysis equipment with conditions of a 10 to 20 mg sample, a 20° C./minheating rate from 25 to 600° C. in a nitrogen atmosphere. Thedetermination is based on the fraction of unvolatilized mass at 600° C.In samples analyzed here the organic mass fraction is completelyvolatilized at temperatures of 400° C. or below, and the residual mass(metal component) is constant from to 400 to 600° C.

Cluster Core Radius

This is a microscopic parameter describing the metal core size. Forinstance, with gold complexes it is calculated from a spherical modelrelating the core size to the alkanethiol monolayer coverage (see R. H.Terrill et al, J. Amer. Chem. Soc. 117, 12537 (1995)). The model assumesa hexagonal closest packed arrangement of surface gold atoms with two ofthree gold surface atoms chemically bonded to individual organic ligandsof the ligand shell. Adaptation to other conductive metals is within theskill of the art, once the skilled worker is in possession of theteachings herein.

Plasmon Absorptivity

This is an optical parameter characterizing the metal core size of theparticles in solution. For the gold based particles in dilute chloroformsolution (0.4 mg/g chloroform), it is the 507 nm absorptivity normalizedto a gram-atom of gold in the particle.

Resistance of Sensor Device

This parameter is a baseline from which modulations driven by vaporexposures occur. It is determined by both the resistivity of theparticle thin film and the geometry of the electrode. The sensor deviceis fabricated by deposition of a particle thin film onto interdigitalmicroelectrodes. The microelectrodes comprise a gold electrode arrayfabricated on a 7×12.5×1 mm quartz substrate and consist of 50 fingerpairs with the following dimensions: spacing, 15 microns; finger width,15 microns; overlap length, 4800 microns; electrode thickness, 1500 A.This device is mounted on a brass plate heated to 120° C. and coatedwith a 0.2 to 0.4 micron thick Au:C₈(1:1) film. Film deposition can beaccomplished using an airbrush technique. In this case a 10 mg/mlchloroform solution is sprayed using a fine nozzle setting for 16 passesof one second duration. This produces a film with a thickness of 0.2micron. Device resistances utilizing various particles as filmsdeposited on the electrodes are entered in Table 1. Based on the abovefilm thickness and electrode geometry, the film resistance (ohms) can beconverted into a bulk material resistivity (ohm cm) by multiplying by afactor of 0.63. If the self-assembly film deposition technique is used,this conversion factor is not applicable since it incorporates a filmthickness parameter.

Sensor Response to Vapors

This test refers to the magnitude of the modulation from the baselineresistance caused by exposure of the sensor device to a vapor.Resistivity (conductivity) changes in the sensor are measured using anAC technique. A square wave potential of 5 volts and a frequency of 100hz is used to excite the microelectrode. The resulting current in themicroelectrode is processed using a current-to-voltage converter circuitfollowed by a precision rectifier and low-pass filter. Thus, themagnitude of the 100 hz AC microelectrode current is converted into aproportional DC voltage. This voltage is converted to a frequency (usinga V/F converter to allow data acquisition over a wide dynamic rangeusing a computerized frequency counter). This measurement scheme allowsthe microelectrode current to be measured, displayed, and stored with atime resolution of one second and a relative current resolution of0.001%. The sensor is mounted in a gold plated aluminum cell designedfor low dead volume (i.e., <0.5 cc) with intake and exhaust portspositioned immediately above the electrode and connected to the circuitby way of pogo pin contacts to the contact pad of the electrode. Vaporsof toluene, tetrachloroethylene, 1-propanol and water are generated frombubblers at 15° C., diluted with dry air to desired concentrations anddelivered to the cell on an alternating exposure-purge schedule by acomputer controlled vapor generator. An example of a typical sensor'sresponses to toluene vapor at 11,000 and 2.7 ppmv are illustrated inFIGS. 5 and 7, and the concentration dependencies are presented in FIG.6. At the low end of the concentration range the dependence becomesnearly linear. As a single parameter to characterize and rank theresponses of the different metal cluster films to individual vapors, theslope of the plot in FIG. 6 at a P/Po value of 0.10 is very useful. Thisparameter characterizes the sensitivity as well as the selectivity of aparticular film of particles. A tabulation of this parameter ispresented in Table 2 for the above cited vapors with sensors utilizingvarious particles.

Further information concerning embodiments and advantages of the claimedinvention is set forth in the following examples and companion Tables 1and 2. The characterizing features and test results reported in thoseTables conform generally to the preceding discussion of test proceduresand parameters.

EXAMPLE 1 (Au:SC8(1:1))

Solutions of: 4.56 g tetraoctylammonium bromide ((C₈H₁₇)₄NBr) in 167 mltoluene; 0.8025 g (2.04 mmol) hydrogen tetrachloroaurate (III)trihydrate (HAuCl₄.3H₂O) in 62.5 ml distilled water; 0.297 g (2.03 mmol)1-octanethiol (C₈H₁₇SH) in 2 ml toluene; and 0.7870 g sodium borohydride(NaBH₄) in 52.5 ml distilled water are prepared. With rapid stirring theAUCl₄/water solution is slowly added to the (C₈H₁₇)₄NBr/toluenesolution. After 2 minutes the (C₈H₁₇SH)/toluene solution is addedfollowed by the slow addition of the NaBH₄/water solution with veryrapid stirring. The vigorous stirring is continued for 3 hr. The toluenephase is then separated and concentrated to a 10 ml volume at reducedpressure. The product is precipitated by dropwise addition into 800 mlrapidly stirred ethanol. After settling for several hours at 10° C., thesupernate is decanted, and product is collected by centrifugationfollowed by washing with fresh ethanol and drying. This crude product isredissolved in 4 ml toluene and reprecipitated by dropwise addition into200 ml rapidly stirred ethanol. After standing 12 hr at 10° C., theproduct is collected by centrifugation, washed with fresh ethanol andvacuum dried. The yield is 0.39 g. The gold mass fraction, core radiusand 507 nm optical absorptivity characterization are presented inTable 1. A sensor fabricated by an airbrushed deposition of a thin filmof this Au:C₈(1:1) particulate onto an interdigital microelectrodedisplays a baseline device resistance of 1.3 MΩ and 0.1 P/Po normalizedresistance responses to toluene, tetrachloroethylene, 1-propanol andwater vapors of 1.82, 1.45, −0.0514 and −0.0036 respectively (see Table2).

EXAMPLE 2 (Au:SC8(1:3))

The Au:SC8(1:3) particle is prepared, fabricated into a sensor andtested as in Example 1 except utilizing a 1:3 mole ratio ofHAuCl₄:C₈H₁₇SH. Characterization and sensor test results are presentedin Tables 1 and 2.

EXAMPLE 3 (Au:SC8(3:1))

The Au:SC8(3:1) particle is prepared, fabricated into a sensor andtested as in Example 1 except utilizing a 3:1 mole ratio ofHAuCl₄:C₈H₁₇SH. Characterization and sensor test results are presentedin Tables 1 and 2

EXAMPLE 4 (Au:SC8(5:1))

The Au:SC8(5:1) particle is prepared, fabricated into a sensor andtested as in Example 1 except utilizing a 5:1 mole ratio ofHAuCl₄:C₆H₁₇SH. Characterization and sensor test results are presentedin Tables 1 and 2.

EXAMPLE 5 (Au:SC6(1:3))

The Au:SC6(1:3) particle is prepared, fabricated into a sensor andtested as in Example 1 except utilizing 1-hexanethiol (C₆H₁₃SH) in placeof C₈H₁₇SH and a 1:3 mole ratio of HAUCl₄:C₆H₁₃SH. Characterization andsensor test results are presented in Tables 1 and 2.

EXAMPLE 6 (Au:SC6(1:1))

The Au:SC6(1:1) particle is prepared, fabricated into a sensor andtested as in Example 1 except utilizing 1-hexanethiol (C₆H₁₃SH) in placeof C₈H₁₇SH and a 1:1 mole ratio of HAuCl₄:C₆H₁₃SH. Characterization andsensor test results are presented in Tables 1 and 2.

EXAMPLE 7 (Au:SC6(3:1))

The Au:SC6(3:1) particle is prepared, fabricated into a sensor andtested as in Example 1 except utilizing 1-hexanethiol (C₆H₁₃SH) in placeof C₈H₁₇SH and a 3:1 mole ratio of HAuCl₄: C₆H₁₃SH. Characterization andsensor test results are presented in Tables 1 and 2.

EXAMPLE 8 (Au:SC6(5:1)).

The Au:SC6(5:1) particle is prepared, fabricated into a sensor andtested as in Example 1 except utilizing 1-hexanethiol (C₆H₁₃SH) in placeof C₈H₁₇SH and a 5:1 mole ratio of HAuCl₄:C₆H₁₃SH. Characterization andsensor test results are presented in Tables 1 and 2.

EXAMPLE 9 (Au:SC4(1:1))

The Au:SC4(1:1) particle is prepared, fabricated into a sensor andtested as in Example 1 except utilizing 1-butanethiol (C₄H₉SH) in placeof C₈H₁₇SH and a 1:1 mole ratio of HAuCl₄:C₄H₉SH. Characterization andsensor test results are presented in Tables 1 and 2.

EXAMPLE 10 (Au:SC12(1:1))

The Au:SC12 (1:1) particle is prepared, fabricated into a sensor andtested as in Example 1 except utilizing 1-dodecanethiol (C₁₂H₂₅SH) inplace of C₈H₁₇SH and a 1:1 mole ration of HAuCl₄:C₁₂H₂₅SH.Characterization and sensor test results are presented in Tables 1 and2.

EXAMPLE 11 (Au:SC12(3:1))

The Au:SC12(3:1) particle is prepared, fabricated into a sensor andtested as in Example 1 except utilizing 1-dodecanethiol (C₁₂H₂₅SH) inplace of C₈H₁₇SH and a 3:1 mole ratio of HAuCl₄:C₁₂H₂₅SH.Characterization and sensor test results are presented in Tables 1 and2.

EXAMPLE 12 (Au:SC12(5:1))

The Au:SC12(5:1) particle is prepared, fabricated into a sensor andtested as in Example 1 except utilizing 1-dodecanethiol (C₁₂H₂₅SH) inplace of C₈H₁₇SH and a 5:1 mole ratio of HAuCl₄:C₆H₁₃SH.Characterization and sensor test results are presented in Tables 1 and2.

EXAMPLE 13 (Au:SC12(8:1))

The Au:SC12(8:1) particle is prepared, fabricated into a sensor andtested as in Example 1 except utilizing 1-dodecanethiol (C₁₂H₂₅SH) inplace of C₈H₁₇SH and a 8:1 mole ratio of HAuCl₄:C₁₂H₂₅SH.Characterization and sensor test results are presented in Tables 1 and2.

EXAMPLE 14 (Au:SC16(5:1))

The Au:SC16(5:1) particle is prepared, fabricated into a sensor andtested as in Example 1 except utilizing hexadecanethiol (C₁₆H₃₃SH) inplace of C₈H₁₇SH and a 5:1 mole ratio of HAuCl₄:C₁₆H₃₃SH.Characterization and sensor test results are presented in Tables 1 and2.

EXAMPLE 15 (Au:St-C12(1:3))

The Au:St-C12(1:3) particle is prepared, fabricated into a sensor andtested as in Example 1 except utilizing a mixture of tertiary andprimary decanethiol (C₁₆H₃₃SH) in place of C₈H₁₇SH and a 1:3 mole ratioof HAuCl₄:C₁₂H₂₅SH. Characterization and sensor test results arepresented in Tables 1 and 2.

EXAMPLE 16 (Au:St-C12(1:1))

The Au:St-C12(1:1) particle is prepared, fabricated into a sensor andtested as in Example 1 except utilizing a mixture of tertiary andprimary decanethiol (C₁₆H₃₃SH) in place of C₈H₁₇SH and a 1:1 mole ratioof HAuCl₄:C₁₂H₂₅SH. Characterization and sensor test results arepresented in Tables 1 and 2.

EXAMPLE 17 (Au:SCPh(1:1))

The Au:SCPh(1:1) particle is prepared, fabricated into a sensor andtested as in Example 1 except utilizing benzyl mercaptan (C₆H₅CH₂SH) inplace of C₈H₁₇SH and a 1:1 mole ratio of HAuCl₄:C₆H₅CH₂SH.Characterization and sensor test results are presented in Tables 1 and2.

EXAMPLE 18 (Au:SC2Ph(1:1))

The Au:SCPh2(1:1) particle is prepared, fabricated into a sensor andtested as in Example 1 except utilizing phenethyl mercaptan(C₆H₅CH₂CH₂SH) in place of C₈H₁₇SH and a 1:1 mole ratio ofHAuCl₄:C₆H₅CH₂CH₂SH. Characterization and sensor test results arepresented in Tables 1 and 2.

EXAMPLE 19 (Au:SCPhOCH3(1:1))

The Au:SCPhOCH3(1:1) particle is prepared, fabricated into a sensor andtested as in Example 1 except utilizing 4-methoxybenzyl mercaptan(CH₃OC₆H₄CH₂SH) in place of C₈H₁₇SH and a 1:1 mole ratio ofHAuCl₄:CH₃OC₆H₄CH₂SH. Characterization and sensor test results arepresented in Tables 1 and 2.

EXAMPLE 20 (Au:SPh(2:5))

The Au:SPh(2:5) particle is prepared as in Example 1 except utilizingthiophenol (C₆H₅SH) in place of C₈H₁₇SH and a 2:5 mole ratio ofHAuCl₄:C₆H₅SH. After the first precipitation this product ischaracterized by a low solubility in most organic solvents (chloroform,acetone, ethyl ether, tetrahydrofuran, toluene), and a secondreprecipitation was not performed. A moderate solubility was found inchlorobenezene which was sufficient for the deposition procedure on thesensor electrode. The conductivity and vapor sensitivity test resultsare presented in Table 2.

EXAMPLE 21 (Au:SCPhOH(2:5))

Solutions of: 0.2983 g (0.757 mmol) HAuCl₄.3H₂O, 0.2285 g (1.812 mmol)4-hydroxythiophenol (HOC₆H₄SH) and 3 ml acetic acid in 150 ml methanol;and 0.4510 g (11.9 mmol) sodium borohydride (NaBH₄) in 30 ml distilledwater are prepared. The NaBH₄/water solution is added dropwise over a 5min period to the HAuCl₄/HOC₆H₄SH/methanol solution with rapid stirring.Stirring is continued for 45 min. The reaction mixture is thenconcentrated at reduced pressure (20mm/35° C.) to a 2 to 3 ml volume andprecipitated into 150 ml distilled water. The crude product is thenwashed with three 50 ml portions of water. The crude product isdissolved in 3 ml acetone and precipitated into 150 ml ethyl ether. Thisproduct is collected by centrifugation and dried. The yield is 63.5 mg.The conductivity and vapor sensitivity tests are presented in Table 2.

EXAMPLE 22 (Au:NC12(1:11))

Solutions of 0.2347 g (0.596 mmol) HAuCl₄.3H₂O in 50 ml distilled water;1.1930 g (6.44 mmol) 1-dodecylamine (C₁₂H₂₅NH₂) in 50 ml toluene; and0.3592 g (9.50 mmol) sodium borohydride (NaBH₄) in 50 ml distilled waterare prepared. The C₁₂H₂₅NH₂/toluene solution is added to theHAuCl₄/water solution with rapid stirring. After 5 min, a thicksuspension is formed and the NaBH₄/water solution is added dropwise overa 10 min period. The rapid stirring is continued for 12 hr. The toluenephase is then separated and concentrated to a 3 ml volume at reducedpressure (20mm/35° C.). The product is precipitated by dropwise additioninto 225 ml of rapidly stirred methanol, collected by centrifugationafter standing for 3 hr and vacuum dried. The yield is 4.7 mg. Amultiplicity of these particles is deposited as a thin film on thesensor electrode by way of the air brush technique using a chloroformsolution. The conductivity and vapor sensitivity tests are presented inTable 2.

EXAMPLE 23 (Au:NC16(1:11))

The Au:NC16(1:11) particle is prepared and fabricated into a sensor asin Example 22 utilizing an equivalent molar quantity of 1-hexadecylamine(C₁₆H₃₃NH₂) in place of C₁₂H₂₅NH₂. An improved particle yield of 54.2 mgis obtained. The conductivity and vapor sensitivity tests are presentedin Table 2.

EXAMPLE 24 (Au:NC18(1:11))

The Au:NC18(1:11) particle is prepared and fabricated into a sensor asin Example 22 utilizing an equivalent molar quantity of 1-octadecylamine(C₁₈H₃₇NH₂) in place of C₁₂H₂₅NH₂. A further improved yield of 133.2 mgis obtained. The conductivity and vapor sensitivity tests are presentedin Table 2.

EXAMPLE 25 (Au:SC6(3:1)/Fluoropolyol Codeposition)

A solution of 17.9 mg Au:C6(3:1) particle materials (Example 7) and 2.6mg FPOL (fluoroalcohol polymer investigated for dimethylmethyphosphonate (DMMP) sorption: see Snow, A. W. et al, J. Appl. Poly.Sci., 43, 1659 (1991)) are codissolved in 2.40 g chloroform. A sensorfabricated by an airbrushed deposition of a thin film of this polymercluster composite onto an interdigital microelectrode (@120°) displays abaseline resistance of 0.018 megohms and 0.1 P/Po normalized resistanceresponses to toluene, 1-propanol and water vapors of 1.17, 0.258 and0.0265 respectively (see Table 2). Sensitivity to DMMP at 0.0041 P/Po is57 times greater than of the Au:C6(3:1) without the fluoropolyol.

EXAMPLE 26 (Au:SC4(1:1) Self-Assembled Deposition)

An electrode and quartz substrate is surface activated by chloroformimmersion cleaning, immersion in a 5% sodium hydroxide solution for 10min, thorough rinsing with distilled water and drying 2 hr at 120° C.Solutions of: 5.0 mg AuSC4(1:1) precursor materials (Example 9) in 1.00g chloroform; 13.6 mg 1,8-octanedithiol in 1.02 g chloroform; and 6.0 mg3-(dimethoxymethysilyl)-1 propanethiol in 0.5 g hexane (thiol-silanecoupling agent solution) are prepared. After 2-hour drying at 120° C.the electrode is immediately immersed in the thiol-silane coupling agentsolution for 1 hour. After removing the electrode and rinsing withhexane, it is immersed in the octanedithiol solution for 5 min. Theactivated device surface (electrode and substrate) is then rinsed withchloroform and alternately immersed in the Au:SC4(1:1) precursormaterial and octanedithiol solutions for a total of 21 cycles. Eachcycle results in a conductivity increment corresponding to a currentincrease of approximately 400 nA. The deposited film is very uniform andnot removable by immersion in a variety of organic solvents or by mildabrasion with a cotton swab. The conductivity and vapor sensitivitytests are presented in Table 2.

EXAMPLE 27 (Au:SC4(1:1) Self-Assembled Deposition)

An electrode and quartz substrate is surface activated by chloroformimmersion cleaning followed by a reduced pressure oxygen plasma surfacetreatment utilizing a standard laboratory plasma cleaner. This treatmentconsists of three 5 sec leakages of small volumes of air at one minuteintervals into an evacuated chamber containing the sensor electrodeunder a plasma discharging electric field. The deposition of theAu:SC4(1:1) particles then proceeds with the immersion in thethiol-silane coupling agent solution follow by the alternate immersionsin the dithiol and Au:SC4(1:1) precursor material solutions as describedin Example 26.

EXAMPLE 28 (Au:SC4(1:1) Self-Assembled Deposition)

The self-assembled deposition procedure in Example 27 is modified bysubstituting a 3-(trimethoxysilyl)-1-propanethiol coupling agent for the3-(dimethoxymethylsilyl)-1-propanethiol coupling agent. The conductivityand vapor sensitivity test of this sensor are presented in Table 2.

EXAMPLE 29 (Au:SC6(1:1) Self-Assembled Deposition)

A sensor is fabricated with a self-assembled Au:SC6(1:1) depositionfollowing the procedure in Example 26 except that Au:SCG(1:1) particlesare substituted for the Au:SC4(1:1) particles and the number ofdeposition cycles is 22. The conductivity and vapor sensitivity tests ofthis sensor are presented in Table 2.

EXAMPLE 30 (Au:SC8(1:1) Self-Assembled Deposition)

A sensor is fabricated with a self-assembled Au:SC8(1:1) depositionfollowing the procedure in Example 26 except that Au:SC8(1:1) particlesare substituted for the Au:SC4(1:1) particles, and the number ofdeposition cycles is 28. The conductivity and vapor sensitivity tests ofthis sensor are presented in Table. 2.

EXAMPLE 31 (Condensed Phase)

An Au:SC8(1:1) sensor is fabricated as described in Example 1 andutilized to detect toluene dissolved in distilled water. The sensor isimmersed in distilled water and then in toluene/water solutions atconcentrations of 0.052 and 0.0052 weight percent. Responses of thesensor are measure under DC conditions with a constant bias potential of0.105 V. The immersion in the distilled causes an increase in thebaseline current from 40 to 180 nA. Subsequent immersion in the 0.052weight % toluene/water solution causes the baseline current to decreaseto 50 nA (a 72% decrease in current). Reimmersion in distilled waterreturns the baseline current to 180 nA. Four repetitions of thesealternating immersions produce the same result. The same experimentconducted with the 0.0052 weight % toluene/water solution resulted in a12% decrease in the sensor current.

EXAMPLE 32 (Condensed Phase)

An Au:SC4(1:1) sensor is fabricated as described in Example 28 andutilized to detect toluene dissolved in distilled water. The sensor isimmersed in distilled water and then in toluene/water solutions at aconcentration of 0.052 weight percent. Responses of the sensor aremeasured under DC conditions with a constant bias potential of 0.05 V.The immersion in the distilled causes an decrease in the baselinecurrent from 6600 to 4400 nA. Subsequent immersion in the 0.052 weight %toluene/water solution causes the baseline current to further decreaseto 4200 nA (a 4.5% decrease in current). Reimmersion in distilled waterreturns the baseline current to 4400 nA. Three repetitions of thesealternating immersions produce the same result.

TABLE 1 Stabilized Particle Characterization R_(Core) ε₅₀₇ ExampleParticle W_(Au) (nm) (1/mol Au cm) 1 Au:SC8(1:1) 0.807 1.11 2190 2Au:SC8(1:3) 0.751 0.63 1640 3 Au:SC8(3:1) 0.841 1.53 2410 4 Au:SC8(5:1)0.904 3.01 2790 5 Au:SC6(1:3) 0.792 0.66 1560 6 Au:SC6(1:1) 0.824 0.961970 7 Au:SC6(3:1) 0.884 1.61 2510 8 Au:SC6(5:1) 0.919 2.92 2700 9Au:SC4(1:1) 0.879 1.24 2200 10 Au:SC12(1:1) 0.755 1.14 2260 11Au:SC12(3:1) 0.796 1.56 2550 12 Au:SC12(5:1) 0.871 2.97 2830 13Au:SC12(8:1) 0.895 3.61 2970 14 Au:SC16(5:1) 0.835 2.83 2750 15Au:St-C12(1:3) 0.828 1.25 2280 16 Au:St-C12(1:1) 0.871 2.90 2780 17Au:SCPh(1:1) 0.838 1.18 2100 18 Au:SC2Ph(1:1) 0.810 1.04 2346 19Au:SC1PhOCH3(3:1) 0.796 1.56 2550

TABLE 2 Sensor Response to Vapors Ro (DR/Ro)/(P/Po)_(0.10) ExampleParticle (Megohm) Tol TCE PrOH H₂O 1 Au:SC8(1:1) 1.3 1.82 1.45 −0.051−0.0036 2 Au:SC8(1:3) 300 0.38 0.30 −0.37 −0.019 3 Au:SC8(3:1) 0.23 1.691.42 −0.0077 −0.0006 4 Au:SC8(5:1) 0.11 1.58 1.40 0.22 0.020 5Au:SC6(1:3) 20 1.31 1.07 −0.012 −0.0064 6 Au:SC6(1:1) 0.29 1.72 1.550.077 −0.0057 7 Au:SC6(3:1) 0.0066 1.60 1.45 −0.0035 −0.0082 8Au:SC6(5:1) 0.0025 0.87 1.04 0.064 0.0063 9 Au:SC4(1:1) 0.0047 0.80 0.940.0052 0.0013 10 Au:SC12(1:1) 140 0.48 0.36 −0.096 −0.018 11Au:SC12(3:1) 32 0.98 0.78 −0..015 −0.0060 12 Au:SC12(5:1) 1.4 1.42 1.160.113 0.015 13 Au:SC12(8:1) 0.57 1.42 1.18 0.194 0.021 14 Au:SC16(5:1)110 0.44 0.32 0.0026 0.0009 15 Au:St-C12(1:3) 0.27 1.26 — 0.25 0.019 16Au:St-C12(1:1) 0.0013 0.66 — 0.053 0.0064 17 Au:SCPh(1:1) 0.0003 0.11 —0.0311 0.0050 18 Au:SC2Ph(1:1) 0.031 1.85 1.58 0.045 −0.0001 19Au:SC1PhOCH3(3:1) 0.0026 0.624 — 0.14 0.0236 20 Au:SPh(2:5) 0.0011−0.049 — −0.0052 −0.0001 21 Au:SPhOH(2:5) 990 −0.0030 — −0.027 −0.30 22Au:NC12(1:11) 0.0016 0.87 — 0.34 0.11 23 Au:NC16(1:11) 0.069 1.26 — 0.130.015 24 Au:NC18(1:11) 0.29 1.03 — 0.12 0.023 25 Au:SC6(3:1)/FPOL 0.0181.17 — 0.285 0.0265 26 Au:SC4(1:1) S-A 0.0049 0.47 — 0.085 0.019 27Au:SC4(1:1) S-A 0.0071 — — — — 28 AuSC4(1:1) S-A 0.0056 0.51 — — — 29AuSC6(1:1) S-A 11 0.82 — 0.34 0.17 30 AuSC8(1:1) S-A 1.3 0.74 — 0.340.15

The invention described herein is susceptible of many modifications andvariations within its scope, and in particular extends to the use of anyone or more of the singular and several features of the foregoingdescription and accompanying drawings and their equivalents.

What is claimed is:
 1. A method for investigating a target environmentto determine whether or in what amount a chemical species may be presenttherein, which comprises (a) exposing to said environment an article ofmanufacture comprising a multiplicity of particles in close-packedorientation, said particles having a core of conductive metal orconductive metal alloy and deposited thereon a ligand which is capableof interacting with said species such that a property of saidmultiplicity of particles is altered; (b) subjecting said multiplicityof particles to conditions sufficient for said property to be exhibited;and (c) monitoring said property to determine whether there is, or theamount of, any change as an indication of whether, or in what amount,said species is present.
 2. A method as defined in claim 1, wherein saidcore comprises silver, gold, platinum or palladium or an alloy of two ormore of such metals.
 3. A method as defined in claim 1, wherein saidligand shell comprises a substance which is capable of interacting withsaid species such that the conductivity of said multiplicity ofparticles is altered.
 4. A method as defined in claim 1, wherein saidligand comprises a thiol or an amine.
 5. A method as defined in claim 1,wherein in each said particle the core is of size from 0.8 to 40 nm inmaximum dimension and the ligand shell is of thickness from 0.4 to 4.0nm.
 6. A method as defined in claim 1, wherein the particles aresubstantially spherical.
 7. A method as defined in claim 1, wherein thespecies, when present, can be detected at an amount of 100 ppm or less.8. A method for investigating a target environment to determine whether,or in what amount, a chemical species may be present, which comprises(a) exposing to said environment a multiplicity of particles having acore of conductive metal or conductive metal alloy, in each saidparticle the core being of from 0.8 to 40.0 nm in maximum dimension, anddeposited on said core a ligand shell, of thickness from 0.4 to 4.0 nm,which is capable of interacting with said species such that theelectrical conductivity of said multiplicity of particles is altered;(b) measuring the electrical conductivity of said multiplicity ofparticles to determine whether there has been, or the amount of, anychange in such conductivity compared to the electrical conductivity ofsuch particles not exposed to said environment.
 9. A method as definedin claim 8, which comprises exposing the multiplicity of particles tosaid environment such that when the species is present the ligand shellabsorbs said species and swells, with the result that the conductivityof the multiplicity of particles is changed.
 10. A method as defined inclaim 8, which comprises exposing the multiplicity of particles to saidenvironment such that when the species is present the electronic chargedistribution of the ligand shell is altered, with the result of theconductivity of the multiplicity of particles is changed.
 11. A methodas defined in claim 8, which further comprises comparing (a) themeasurement of the conductivity of said multiplicity of particlesexposed to said environment and (b) a contemporaneous measurement of theelectrical conductivity of a comparable multiplicity of such particlesnot exposed to said environment.